Device for the ratiometric measurement of sensor signals

ABSTRACT

Device for the ratiometric measurement of sensor signals using an analog/digital converter whose supply voltage is generated by means of a voltage controller, having an operational amplifier which is connected as a voltage follower and whose noninverting input is supplied with the sensor reference voltage and whose output is connected to the supply voltage input via a voltage divider whose tap is fed back to the inverting input of the voltage follower.

BACKGROUND OF THE INVENTION Field of the Inventiuon

The invention relates to a device for the rationmetric measurement ofsensor signals.

The demands on the measurement accuracy of sensors in motor vehicleelectronics are becoming ever higher. Many measured values aretransmitted from the sensor to the processing microcontroller as analogDC voltages Vmeβ (0 . . . 5V+0 . . . 100%). The microcontrollerdigitizes them using an analog/digital converter ADC (e.g. 10 bit ADC: 0. . . 5V+0 . . . 1023 steps). In an analog/digital converter, themeasured voltage Vmeβ to be converted is set in relation to a referencevalue VAref, which is supplied to the analog/digital converter ADC as ananalog reference voltage. To convert the analog value to a digital valueas accurately as possible, it is therefore desirable for the sensor Sand the analog/digital converter ADC to use the same reference voltage.

In motor vehicle electronics, this is generally not possible because thereference voltage VAref used for the analog/digital converter ADCintegrated in the microcontroller μC is the supply voltage V5int for themicrocontroller μC (VAref=V5int). Since, however, in the event of ashort circuit between the sensor reference voltage V5ext and ground orthe battery, the microcontroller and hence the engine/transmissioncontrol unit need to remain operational, the supply voltage V5int mustnot be routed to the vehicle wiring harness.

In this context, the notation V5int signifies the internal 5 volt supplyvoltage for the microcontroller μC, while V5ext signifies the 5 voltreference voltage for the sensor or sensors S arranged outside of themotor control unit ST. The supply voltage for microcontroller andsensors is usually 5 V.

A voltage controller SR having a plurality of mutually independentoutputs—V5int, V5ext—and a common reference Ref is generally used, seeFIG. 1. The mutual discrepancy (tracking error of the controller) whichis possible in this context is typically V5int−V5ext=±50 . . . 100 mV.In sensors such as air mass flowmeters, this error (1 . . . 2%) isalready an essential proportion of the total error budget.

SUMMARY OF THE INVENTION

It is therefore the object of the invention to provide a device formeasuring sensor signals which can be used to eliminate or minimize thistracking error.

In the device according to the invention, the reference voltage VAreffor the analog/digital converter ADC is isolated from the supply voltageV5int for the microcontroller μC. It follows—in a certain range aroundthe supply voltage V5int—the sensor reference voltage V5ext, so that thetracking error is largely eliminated in this range. This range isdefined by the maximum permissible difference between the referencevoltage VAref for the analog/digital converter ADC and the supplyvoltage V5int for the microcontroller μC; in a typical microcontrollerused in automobile electronics, it is ±100 V, for example.

An illustrative embodiment in accordance with the invention is explainedin more detail below with the aid of a schematic drawing, in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a circuit used previously for measuring sensor signals,

FIG. 2 shows a circuit according to the invention, and

FIG. 3 shows a graph of the curve of the voltages used V5int, V5ext andVAref.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 shows, in a previously used circuit, a motor control unit ST(indicated as a box) in a motor vehicle internal combustion engine (notshown), having an aforementioned voltage controller SR which is operatedat the battery voltage+U_(B), having two mutually independent outputsfor the voltages V5int and V5ext and having a common reference Ref. Thevoltage V5int is used as the supply voltage for a microcontroller μC andat the same time as a reference voltage VAref for an analog/digitalconverter ADC which is integrated in to the microcontroller μC and whichis supplied with the output signal Vmeβ from a sensor S which is in theform of a potentiometer and is arranged outside of the control unit ST,for the purpose of the ratiometric A/D conversion. The reference voltageV5ext received by the sensor S is the second output voltage from thevoltage controller SR. As already described, this circuit has a trackingerror between V5int and V5ext of, typically, ±50 . . . 10 mV.

FIG. 2 shows an inventive circuit which eliminates this tracking errorin a particular voltage range.

The engine control unit ST is indicated by a dashed line; although thevoltage controller SR shown in FIG. 1 is also present in the enginecontrol unit ST shown in FIG. 2, only its two voltage outputs are shownin this case as inputs for the circuit downstream, for the sake ofclarity: INT for the supply voltage V5int for the internalmicrocontroller μC and EXT for the reference voltage V5ext for theexternal sensor S.

To form the reference voltage VAref for the analog/digital converterADC, there is an operational amplifier IC1 which is connected as avoltage follower with a gain factor of +1 and is supplied with a voltageVaux obtained from the vehicle battery. The noninverting input “+” ofsaid operational amplifier is connected to the voltage input EXT via aresistor R1. The resistor R1 serves as protection for the operationalamplifier IC1 against possible interference voltages which may besuperimposed on the voltage V5ext.

The noninverting input “+” is first connected to the ground connectionGND of the control unit ST via a capacitor C1. The capacitor C1 forms,together with the resistor R1, an additional low pass filter forfiltering relatively high frequency interference. The noninverting input“+” is secondly connected to the voltage input INT via two reverseconnected parallel diodes D1, D2, which means that the voltage (V5ext)applied to the noninverting input “+” is additionally limited to avoltage range V5int ±0.7 V.

The output of the operational amplifier IC1 is connected to the voltageinput INT via a voltage divider which is formed from resistors R2 and R4and whose tap A is connected to the inverting input “−” of theoperational amplifier IC1 via a resistor R3. This tap A provides thereference voltage VAref, which is supplied to the analog/digitalconverter ADC together with the measured voltage Vmeβ tapped off at thesensor S.

The way in which this circuit operates, which is described below, isbased on the voltages and component values specified below.

The voltage range at the output of the operational amplifier IC1 isapproximately 0.4 V to 9.1 V for the assumed supply voltage of Vaux=10V. The feedback via the resistor R3 causes the operational amplifier IC1to attempt to eliminate the voltage difference between its inputs, i.e.to correct the voltage VAref, which is also applied to the invertinginput, to the voltage V5ext applied to the noninverting input.

If, by way of example, the reference voltage V5ext has a tracking errorof −100 mV with respect to the supply voltage V5int, which is intendedto be exactly 5 V, i.e. V5ext=4.9 V (cf. FIG. 3, which plots the voltagecurves for V5int, V5ext and VAref), then these 100 mV must be droppedacross the resistor R4 so that the reference voltage VAref on the tap Aof the voltage divider R4/R2 becomes equal to the reference voltageV5ext. The voltage divider ratio R2/R4 (20 kΩ/0.5 kΩ=40) then means thata voltage of 40*100 mV=4 V must be dropped across the resistor R2,however, i.e. the output of the operational amplifier IC1 is set to avoltage value of 0.9 V.

With the component values given below, the voltage divider and itsconnection to the supply voltage V5int via the input INT produce for thereference voltage VAref a range of approximately VAref=V5int (=5V) ±100mV. In this range, the two voltages V5ext and VAref are of identicalsize: VAref≡V5ext; ratiometric measurement in this range is thereforeensured. One possible error is still provided by the offset voltage ofthe operational amplifier IC1, which may be approximately ±1 . . . 10mV, depending on the type used. However, this error is one order ofmagnitude smaller than the tracking error in previous solutions (±100 mVaround V5int).

If the reference voltage V5ext becomes lower than 4.9 V or higher than5.1 V, then VAref remains constant at approximately 4.9 V or 5.1 V, asshown in FIG. 3, since in this case the operational amplifier output hasalready reached its lower or upper limit value and a smaller or largervalue cannot arise for VAref on account of the voltage divider ratioR2/R4.

In a preferred illustrative embodiment in accordance with FIG. 2, thefollowing components and voltages are used:

D1, D2: DIN4148 R1: 10 kΩ V5int: 5V

C1: 1nF R2: 20 kΩ V5ext: 5 V

IC1: LM324 R3: 10 kΩ Vaux: 10V

R4: 0.5 kΩ

I claim:
 1. A device for ratiometric measurement of sensor signals,comprising: a supply input for receiving a supply voltage; ananalog/digital converter connected to said supply input to receive thesupply voltage, said analog/digital converter connected to receive afirst reference voltage; a sensor for providing an analog output signalthat is supplied to said analog/digital converter, said sensor connectedto receive a second reference voltage; a voltage controller supplyingthe supply voltage to said supply input, and supplying, independentlyfrom the supply voltage, the second reference voltage; a first resistorconnected to receive the second reference voltage; a voltage dividerincluding a second resistor, a third resistor, and a voltage tap betweensaid second resistor and said third resistor; a fourth resistorconnected to said voltage tap of said voltage divider; and anoperational amplifier that is connected as a voltage follower forforming the first reference voltage; said operational amplifier having anoninverting input connected to said first resistor to receive thesecond reference voltage, said operational amplifier having an outputconnected through said voltage divider to said supply input; saidoperational amplifier having an inverting input connected through saidfourth resistor to said voltage tap of said voltage divider; and saidvoltage tap of said voltage divider providing the first referencevoltage.
 2. The device according to claim 1, comprising a low passfilter is connected upstream of said noninverting input of saidoperational amplifier.
 3. The device according to claim 1, whichcomprises two reverse connected parallel diodes connecting saidnoninverting input of said operational amplifier to said supply input.